However, after sitting in the fridge overnight, it was quite tough. It was left uncovered in the fridge. Not sure if this caused it - if yes, what is the best way to keep it in the fridge while the flavours blend?

I make tiramisu from time to time, tough my recipe is much simpler (I use sponge finger instead of cake).

If have noticed that you can separate the ingredients into two categories:

Ingredients that make the tiramisu more liquid:

Eggs

Cofee

Ingredients that make the tiramisu more dry:

Mascarpone

Sugar

sponge fingers

So you have to adjust the relative quantities in order to have the texture that you want. How much cofee you put into the sponge fingers or the cake matters a lot too.

If your problem is that the tiramisu becomes dry after cooling down, then I have to suggestions:

Wrap it up into an impervious container (as mentioned in the other response, to avoid frige smell and evaporation)

The colder the cake, the dryer it will be. So if your cake is too liquid, you might consider putting it into the freezer a few minutes before serving; and if it's too dry you might want to let it warm up a little bit.